1. Field of the Invention
This invention generally relates to a method of manufacturing a thrust plate for a shaft in a dynamic pressure bearing, the shaft having an annular shaft body in which an outer peripheral surface thereof forms a portion of a radial bearing unit. More particularly, the present invention relates to a method of manufacturing a thrust plate that has an annular shape and a central hole formed therein into which the shaft body is fitted, and which has thrust surfaces formed on both end surfaces thereof that form a portion of a thrust bearing unit.
2. Background Information
A recording disk drive device for a hard disk and the like includes a spindle motor for rotatively driving a recording disk, and is concentrically disposed with respect to the recording disk. The spindle motor is primarily comprised of a stationary member to which a stator having an armature coil is fixed, a rotary member that is fixed to a rotor magnet that faces the stator, and a bearing mechanism that supports the rotary member in the stationary member such that the rotary member is freely rotatable with respect thereto.
A hydrodynamic bearing is used as the bearing mechanism in order to achieve higher speeds and lower vibration (noise). The hydrodynamic bearing is comprised of a lubricating fluid such as oil that is disposed in a small gap between the shaft and the sleeve, and a radial/thrust bearing unit that includes dynamic pressure generating grooves that are formed on opposite surfaces.
More specifically, a spindle motor for a hard disk drive in which a dynamic pressure bearing is used has been disclosed in Japanese Published Patent Application 2000-134897 and will be described below. This spindle motor is comprised of a stationary member, a rotary member, and a bearing mechanism that is provided therebetween.
The stationary member is comprised of a motor frame 10 that is fixed to the base of a hard disk drive, a cylindrical boss unit that is integral with the motor frame 10 and disposed such that it is concentric therewith, and a sleeve 14 that is fitted into and fixed to the inner peripheral surface of the cylindrical boss section. A stator 20 is fitted around the outer peripheral surface of the boss section and fixed thereto.
The rotary member is comprised of a rotor hub 16, and a shaft 22 that is integral therewith. A recording disk is mounted on the rotor hub 16. Furthermore, an annular rotor magnet 18 is mounted on the inner side of a lower portion of an outer peripheral wall of the rotor hub 16, and faces the stator 20 in the radial direction. The shaft 22 is disposed such that it is capable of rotating inside the sleeve 14, and herringbone shaped dynamic pressure generating grooves are formed on one or both of an outer peripheral surface of the shaft 22 and an inner peripheral surface of the sleeve 14. The gap between both of these opposing surfaces is filled with a lubricating agent such as oil, thus forming a pair of vertically disposed radial dynamic pressure bearing units. A thrust plate (not labeled with a reference numeral) provided on the lower end of the shaft is housed in a lower end large diameter section of the sleeve 14, and a thrust cover 12 is fitted into fixed to the lower end opening of a boss on the motor frame 10 so as to close the lower end large diameter section of the sleeve 14. Herringbone shaped or spiral shaped dynamic pressure generating grooves are formed on one or both of the upper surface of the thrust plate and a thrust surface of the sleeve 14 that opposes the upper surface of the thrust plate. The gap between these opposing surfaces is filled with a lubricating agent to thereby form an upper thrust dynamic pressure bearing unit. Herringbone shaped or spiral shaped dynamic pressure generating grooves are formed on one or both of the bottom surface of the thrust plate and the thrust cover 12 that opposes the bottom surface of the thrust plate, and a gap between these opposing surfaces is filled with a lubricating agent to thereby form an lower thrust dynamic pressure bearing unit.
In a dynamic pressure bearing spindle motor constructed in this manner, when the coil of the stator 20 is supplied with electricity, rotational torque is generated by the electromagnetic interaction between a rotating magnetic field of the stator 20 and a multipolar magnetic field of the rotor magnet 18, thereby rotating a rotary member which includes the rotor hub 16, the shaft 22 and a rotation load (recording disk). During this rotation, the radial load of the rotary member is supported by the pair of vertically disposed radial dynamic pressure bearing units formed between the shaft 22 and the sleeve 14, and the thrust load of the rotary member is supported by the pair of thrust dynamic pressure bearing units formed respectively between the thrust plate and the sleeve 14 and the thrust cover 12.
However, in a dynamic pressure bearing as described above, the dynamic pressure bearing shaft forms both a radial dynamic pressure bearing unit on the shaft body and thrust dynamic pressure bearing units on both surfaces of the thrust plate. This configuration requires a highly precise perpendicular angle between the outer peripheral surface of the shaft body and the planes of the thrust plate. More specifically, the radial gap in the radial bearing unit between the outer peripheral surface of the shaft body and the inner peripheral surface of the sleeve is normally several xcexcm, and the thrust gap in the thrust bearing units between both surfaces of the thrust plate and the sleeve and the thrust cover is normally about 10 xcexcm. Thus, there is a need for the degree of precision in the perpendicular angle of the planes of the thrust plate relative to the axial center line of the shaft to be within several xcexcm or less.
On the other hand, because the end portion of shaft body of the dynamic pressure bearing shaft disclosed in the aforementioned Japanese Published Patent Application 2000-134897 is press fit into and fixed to the central hole of the thrust plate, and the shaft body and the thrust plate are separate components, it will be more difficult to obtain a perpendicular angle between the thrust plate planes relative to the axial center line of the shaft body that it would be when the shaft is manufactured by cutting it from a unitary member and machining it. It is possible, however, to use a tool to secure the proper degree of precision during shaft body press fitting relative to the central hole of the thrust plate. In other words, if a sufficiently high degree of precision in the perpendicular angle of the central hole in relation to the thrust plate planes can be achieved (i.e., a degree of precision of several xcexcm or less), a dynamic pressure bearing with good rotation run-out precision can be obtained.
Accordingly, we will now look at the problems with the perpendicular angle of the central axis in relation to the surface planes of the thrust plate. In situations in which a spindle motor rotatively drives a recording disk that is, for example, 3.5 inch in diameter, a thrust plate having an outer diameter of 7 to 8 mm, an inner diameter of 4 mm, and a thickness of 2 to 3 mm will be employed, and is normally obtained by press forming.
A machining process that uses press cutting (shearing) to obtain an inner peripheral surface of a blank intermediate will produce sheared surfaces, ruptured surfaces, and/or turned up edges (burrs) on the press-cut surfaces. Thus, at the last step of machining, it will be necessary to both finish the inner and outer peripheral surfaces, and to finish both end surfaces. In this situation, it is difficult to reliably obtain a sufficient degree of precision even when the inner and outer peripheral surfaces are finished, and thus it will be difficult to reliably obtain a sufficient degree of precision in the perpendicular angle of the central axis relative to the surface planes of the thrust plate.
Next, a machining process which uses a coining step to obtain an inner peripheral surface of a blank intermediate will be described. This method of manufacturing includes a blanking step, a dual-side polishing step, a barrel step, a finish polishing step, a coining step, and a flattening step. In the blanking step, an annular blank intermediate is pressed cut from a plate-like work piece. Then, a blank intermediate having a degree of precision in its inner peripheral surface is obtained by means of the dual-side polishing step, the barrel step, and the finish polishing. In the coining step, the blank intermediate is put into a coining die and surface-pressed to improve, primarily, the precision of the inner diameter and the perpendicular angle of both end surfaces with respect thereto. More specifically, a pin having an outer diameter finished with a good degree of precision is placed in the center of the coining die, the pin is inserted into the central hole of the blank intermediate, and the blank intermediate is pressed from both surfaces in this state. Then, the blank intermediate is slightly squeezed, thereby causing a portion thereof to flow toward the inner and outer diameters. As a result, the blank intermediate will have an inner and outer diameter that corresponds to the coining die. In the flattening step, both surfaces of the blank intermediate are pressed to a predetermined height.
In the coining step, however, the material that forms the blank intermediate will not flow in the radial directions in a uniform manner, and thus it will be easy to produce non-uniform surfaces. Thus, the degree to which both end surfaces of the blank intermediate are parallel to each other will be poor, and there will be large fluctuations thereon. In addition, the fluctuations will not sufficiently eliminated in the pressing step, with the result that the heights of products will differ.
It is an object of the present invention to provide a method of manufacturing a thrust plate used in a dynamic pressure bearing that employs a shaft in which the shaft body thereof is fitted into the thrust plate, the thrust plate having a reliable and highly precise degree of run-out precision (perpendicular angle) in the end surfaces thereof relative to a central axis thereof, which thus enhances the bearing performance of the dynamic pressure bearing.
According to one aspect of the present invention, a method of manufacturing a thrust plate for a shaft in a dynamic pressure bearing is disclosed in which the shaft includes an annular shaft body in which an outer peripheral surface thereof forms a portion of a radial bearing unit, and a thrust plate having a central hole formed therein in which the shaft body is fitted and thrust surfaces on both end surfaces thereof that form portions of thrust bearing units. The method of manufacturing include a blanking step in which a plate-like work piece is press-cut to obtain an annular blank intermediate, an end surface polishing step in which both end surfaces of the blank intermediate are polished, and a shaving step in which an inner hole and an outer periphery of the polished blank intermediate are simultaneously press-cut to shave off surfaces thereof.
In this method, a high degree of precision can be reliably obtained in the inner peripheral surfaces because the inner and outer peripheral surfaces of the blank intermediate are shaved off. Moreover, unlike in prior art methods of manufacturing which use a coining step, the degree to which both surfaces of the thrust plate are parallel to each other will maintained at a high level. As a result, a high level of run-out precision (perpendicular angle) in the end surfaces of the thrust plate relative to a central axis thereof can be ensured. Note that shaving is a machining process to again shave a press-cut surface created after the shearing process with a similar shearing tool.
According to another aspect of the present invention, the direction in with the blank intermediate is press-cut in the shaving step is identical with that of the blanking step.
Thus, a fully sheared surface is easy to obtain on the inner peripheral surface of the thrust plate, which improves the precision of the inner peripheral surface.
According to another aspect of the present invention, the end surfaces of the blank intermediate are pressed in the shaving step from both sides thereof in the press-cutting direction.
A high degree of run-out precision (perpendicular angle) in end surfaces of the thrust plate can be secured relative to a central axis thereof at a high level because the degree to which both surfaces of the blank intermediate are parallel to each other has been improved by the dual-side polishing step, and both surfaces have been pressed.
According to another aspect of the present invention, press-cutting is performed in the blanking and shaving steps so as to obtain a fully sheared surface on the press-cut cut surfaces.
This improves the degree of precision of the inner peripheral surface of the thrust plate.
According to another aspect of the present invention, a method of manufacturing a shaft for a dynamic pressure bearing comprises the steps of manufacturing the thrust plate according to the present invention, and fitting the shaft body into the central hole of the thrust plate.
This method will allow the perpendicular angle between the central axis of the shaft body and the planes of the thrust plate to have a high degree of precision.
According to another aspect of the present invention, a dynamic pressure bearing includes the shaft for the dynamic pressure bearing according to the present invention, and a hollow cylindrical member having a through-hole formed therethrough in which the shaft for the dynamic pressure bearing passes. The cylindrical member includes a radial inner peripheral surface that faces an outer peripheral surface of the shaft body with a small gap interposed therebetween, and thrust surfaces facing both end surfaces of the thrust plate with small gaps interposed therebetween. In addition, a radial bearing unit includes the outer peripheral surface of the shaft body, the radial inner peripheral surface of the hollow cylindrical member, and a lubricating fluid disposed in the small gap. Furthermore, thrust bearing sections include both end surfaces of the thrust plate, the thrust surfaces of the hollow cylindrical member, and a lubricating fluid disposed in the small gap.
The bearing performance of the dynamic pressure bearing is improved because it uses the shaft manufactured by means of the method of the present invention. More specifically, a shaft manufactured in this way enables high speed rotation.
According to another aspect of the present invention, a spindle motor includes the dynamic pressure bearing according to the present invention, a stator that is non-rotatably disposed with respect to either the shaft of the dynamic pressure bearing or the hollow cylindrical member, and a rotor magnet which generates a rotating magnetic field in cooperation with the stator, the rotor magnet non-rotatably disposed with respect to the hollow cylindrical member if the stator is non-rotatably disposed with respect to the shaft of the dynamic pressure bearing, and non-rotatably disposed with respect to the shaft of the dynamic pressure bearing if the stator is non-rotatably disposed with respect to the hollow cylindrical member.
High speed rotation is enabled because the spindle motor uses a dynamic pressure bearing according to the present invention.
According to another aspect of the present invention, a recording disk drive device includes a housing, a spindle motor according to the present invention fixed inside the housing, a disk shaped recording medium non-rotatably disposed with respect to the shaft of the dynamic pressure bearing or the hollow cylindrical member and capable of recording data, and data access means for writing data to or reading data from a desired location on the recording medium.
Improvement in data writing and reading speeds can be achieved because the recording disk drive device employs a spindle motor according to the present invention.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.